1. Field of the Invention
The invention is directed to carbon nanotube-containing compositions that have increased viscosity and stability. In particular, the invention is directed to methods for manufacturing carbon nanotube films and layers that provide superior electrical properties.
2. Description of the Background
Carbon nanotubes are the most recent addition to the growing members of the carbon family of molecular structures. Carbon nanotubes can be viewed as a graphite sheet rolled up into a nanoscale tube form to produce the so-called single-wall carbon nanotubes (SWNT) Harris, P. F. “Carbon Nanotubes and Related Structures: New Materials for the Twenty-first Century”, Cambridge University Press: Cambridge, 1999. There may be additional graphene tubes around the core of a SWNT to form multi-wall carbon nanotubes (MWNT). These elongated nanotubes have a diameter in the range from few angstroms to tens of nanometers and a length of several micrometers up to millimeters. Both ends of the tubes may be capped with fullerene-like structures such as pentagons.
Carbon nanotubes comprises straight and/or bent multi-walled nanotubes (MWNT), straight and/or bent double-walled nanotubes (DWNT), or straight and/or bent single-walled nanotubes (SWNT), and combinations and mixtures thereof. CNT may also include various compositions of these nanotube forms and common by-products contained in nanotube preparations such as described in U.S. Pat. No. 6,333,016 and WO 01/92381. Carbon nanotubes may also be modified chemically to incorporate chemical agents or compounds, or physically to create effective and useful molecular orientations (see U.S. Pat. No. 6,265,466), or to adjust the physical structure of the nanotubes.
SWNTs can be formed by a number of techniques, such as laser ablation of a carbon target, decomposing a hydrocarbon, and setting up an arc between two graphite electrodes. For example, U.S. Pat. No. 5,424,054 to Bethune et al. describes a process for producing single-walled carbon nanotubes by contacting carbon vapor with cobalt catalyst. Carbon vapor is produced by electric arc heating of solid carbon, which can be amorphous carbon, graphite, activated or decolorizing carbon or mixtures thereof. Other techniques of carbon heating are discussed, such as laser heating, electron beam heating and RF induction heating. Smalley (Guo, T., Nikoleev, P., Thess, A., Colbert, D. T., and Smally, R. E., Chem. Phys. Lett. 243: 1-12 (1995)) describes a method of producing single-walled carbon nanotubes, wherein graphite rods and a transition metal are simultaneously vaporized by a high-temperature laser. Smalley (Thess, A., Lee, R., Nikolaev, P., Dai, H., Petit, P., Robert, J., Xu, C., Lee, Y. H., Kim, S. G., Rinzler, A. G., Colbert, D. T., Scuseria, G. E., Tonarek, D., Fischer, J. E., and Smalley, R. E., Science, 273: 483-487 (1996)) also describes a process for production of single-walled carbon nanotubes in which a graphite rod containing a small amount of transition metal is laser vaporized in an oven at about 1,200° C. Single-wall nanotubes were reported to be produced in yields of more than 70%. U.S. Pat. No. 6,221,330 discloses methods of producing single-walled carbon nanotubes which employs gaseous carbon feedstocks and unsupported catalysts.
Carbon nanotubes have many well known applications (R. Saito, G. Dresselhaus, M. S. Dresselhaus, “Physical Properties of Carbon Nanotubes,” Imperial College Press, London U.K. 1998, or A. Zettl “Non-Carbon Nanotubes” Advanced Materials, 8, p. 443, 1996). Carbon nanotubes can exhibit semiconducting or metallic behavior (Dai, L.; Mau, A. W. M. Adv. Mater. 2001, 13, 899). They also possess a high surface area (400 m2/g for nanotube “paper”) (Niu, C.; Sichel, E. K.; Hoch, R.; Moy, D.; Tennent, H. “High power electrochemical capacitors based on carbon nanotube electrodes”, Appl. Phys. Lett. 1997, 70, 1480-1482), high electrical conductivity (5000 S/cm) (Dresselhaus, M. Phys. World 1996, 9, 18), high thermal conductivity (6000 W/mK) and stability (stable up to 2800° C. in vacuum) (Collins, P. G.; Avouris, P. “Nanotubes for electronics”, Sci. Am. 2000, Dec. 62-69) and good mechanical properties (tensile strength 45 billion pascals).
Films made of carbon nanotubes are known to have surface resistances as low as 102 ohms/square. U.S. Pat. No. 5,853,877, entitled “Method for Disentangling Hollow Carbon Microfibers, Electrically Conductive Transparent Carbon Microfibers Aggregation Film and Coating for Forming Such Film,” describes formation of conductive carbon nanotube films. U.S. Pat. No. 6,221,330, entitled “Processing for Producing Single Wall Nanotubes Using Unsupported Metal Catalysts,” generally describes production of carbon nanotubes for forming conductive films. However, there has been no report in the art on a method for patterning carbon nanotube-containing films.
Coatings comprising carbon nanotubes, such as carbon nanotube-containing films, have been previously described (see U.S. patent application Ser. No. 10/105,623). Such films may have a surface resistance as low as 102 ohms/square and a total light transmittance as high as 95%. The content of carbon nanotubes in these films may be as high as 50%. Carbon nanotubes may also be deposited on a transparent plastic film to form a transparent conductive coating.
Carbon nanotubes deposited on a surface as a thin coating or film can function as electrical conductors or electrodes, catalytic sites, sensors to detect chemicals, energy, motion or contact (as in touch screens); and other functions which exploit the unique properties of this new form of carbon material. However, to utilize thin coating of nanotubes in most applications, the coating of nanotubes is formed as patterns or circuits defining an active area of nanotubes and separating that area from one or more inactive areas.
For a coating of nanotubes to function as an electrode in a resistive-type touch screen, the electrode must be patterned on an electrically insulating substrate. For example, a polymer film such as polyethylene terephthalate PET, can define parts of the nanotube coating that forms an electrically conductive circuit and switch. That coating then responds to the operator's touch when pressed against a second electrode.
Most commercially produced, transparent electrodes are made from metal or metal oxide coatings applied to an optically transparent substrate by, for example, vacuum deposition, chemical vapor deposition, chemical bath deposition, sputtering, evaporation, pulsed vapor deposition, sol-gel methods, electroplating or spray pyrolysis. If desired, these coatings can be patterned with costly photolithographic techniques. This process is difficult and expensive. Scaling up production to cover large areas with electrodes can be almost prohibitively. Further, because coatings are based on a rigid metal oxide, flexible applications which would otherwise be possible with substrates of plastic displays, plastic solar voltaic and wearable electrical circuitry are also not possible.
Carbon Nanotube (CNT) dispersions in water or other common solvents are thermodynamically unstable, meaning they have a high propensity to self assemble into rope structures. Over time, these ropes can increase in diameter or flocculate, ultimately leading to a de-stabilized dispersion, which is undesirable for coating forming uniform thin coatings of CNT on a surface. To form electrically conductive coatings, it is desirable to maintain the CNT particles as small diameter ropes (less than about 30 nm) in the dispersion until a film is formed on the surface and solvent removed. Once the wet film is formed on a surface, it is desirable to encourage the self assembly of the ropes and thereby form a conductive network of ropes on the surface by removing all other materials. But, if the ropes grow in size or assembly by flocculation of ropes in the coating solution, i.e. before the film is formed, then further assembly of the film is compromised and the resulting dry coating exhibits lower surface resistivity at a given mass deposition per unit area. Furthermore, dispersions of small particles and CNT are typically formed from solvents and dispersing aids like surfactants or other additives like polymers. However the additives will also be deposited in the coating as the solvent evaporate and will interfere with formation of the conductive network. This results in sub-optimal electronic performance for the thin film.
Without the use of surfactants or other additives aside from the solvent carrier, CNT dispersions have been found to be kinetically “stable” both at very low concentrations (less than about 100 mg/liter), and at high concentrations (greater than about 3,000 mg/liter). The low concentration range has the viscosity of the liquid phase (typically about 1 cP) largely due to the solvent, such as water or alcohol. The high concentration range has the viscosity of a “paste” or “gel”. At both ends of the concentration spectrum, the CNT dispersions have useful shelf life (greater than about 8 hours) without need for additives such as surfactant or viscosity modifiers.
The low concentration range is suitable for the spray coating of transparent (and non-transparent) conductive films over a broad range of sheet resistance (typically 10 to 109 ohm/square). The low concentration range is also suitable for various continuous web coating techniques (e.g., gravure, Meyer rod, reverse roll, etc.), but the sheet resistance range is limited to higher sheet resistance values (greater than about 104 ohm/square). The latter limitation is due to practical limits on wet coating thickness for low viscosity coating formulations (typically less than about 50 microns) which being very dilute require a relatively thick wet coating to deposit sufficient material on the surface in a single or multiple applications.
The high concentration range is suitable for various continuous web coating techniques (e.g., gravure, Meyer rod, reverse roll, etc.), but this concentration is too high to allow for higher sheet resistance values (greater than about 102 ohm/square) and results in coating with inferior electrical and optical properties compared to those coating made from deposition of the same amount of CNT per unit area from solutions in the low concentration range.
Thus, a need exists for a coating formulation capability that allows for the preparation of dispersions of CNT over the full concentration range (10 to 3,000 mg/liter or so) with useful stability to allow deposition by traditional coating processes.